Technical Abstract:
In 2011 a severe disease outbreak occurred at a rainbow trout farm in western North Carolina, and researchers from the National Center for Cool and Coldwater Aquaculture (NCCCWA) were able to identify bacteria collected from moribund fish as a novel species of Weissella, gram-positive bacteria not previously associated with trout mortalities in North America. The bacteria were subsequently isolated from one additional trout farm located in close proximity to the initial site, which had moved fish from the infected farm prior to identification of the pathogen. In 2012, Weissella sp. was re-isolated from moribund fish on both farms demonstrating a capacity to sustain across seasons within the fish population on the farms. To date, these bacteria have not been found in other areas of the U.S.
In late 2012 workers at the two infected farms completed vaccination of all fish on the farms using a custom-produced bacterin derived from an autogenous culture of Weisella sp. This bacterin was shown to provide significant protection when fish were exposed in laboratory challenge at NCCCWA, but its efficacy on the farm had not been tested. All fish on these two farms as well as other trout farms in the vicinity were vaccinated by IP injection, with the exception of a small number of trout maintained as unvaccinated controls to help evaluate vaccine effectiveness. In 2013, all the trout farms within the county of the original outbreak site, and all trout farms (15) located within counties contiguous to the original site were surveyed specifically for the presence of Weisella sp. in moribund fish. Bacterial isolates putatively identified as Weissella sp. in culture were subsequently identified by 16S rRNA gene sequence analysis. No Weissella sp. cultures were isolated from either of the original outbreak sites, but the bacteria were found at a third location which had not yet vaccinated against the pathogen. One year post-vaccination, vaccinated and unvaccinated trout from the farm with the original outbreak were challenged by IP injection with 8 x 10 exp 6 cfu/ml live Weissella sp. to evaluate whether the fish retained protection against Weissellosis 12+ months after vaccination. Results showed significant protection remained over one year post-vaccination.